Electroluminescent display devices

ABSTRACT

In an active matrix electroluminescent display device having an array of pixels, a drive transistor ( 22 ) and an electroluminescent display element ( 2 ) in each pixel are connected in series between a power line ( 26 ) for supplying or drawing a controllable current to or from the display element and a common potential line. The power line ( 26 ) and the common potential line each comprise a sheet electrode shared between all pixels of the array. In this arrangement, sheet electrodes are used both for the current supply to the EL display element and the current sink. This reduces considerably the line resistance.

This invention relates to electroluminescent display devices,particularly active matrix display devices having thin film switchingtransistors associated with each pixel.

Matrix display devices employing electroluminescent, light-emitting,display elements are well known. The display elements may compriseorganic thin film electroluminescent elements, for example using polymermaterials, or else light emitting diodes (LEDs) using traditional III-Vsemiconductor compounds. Recent developments in organicelectroluminescent materials, particularly polymer materials, havedemonstrated their ability to be used practically for video displaydevices. These materials typically comprise one or more layers of asemiconducting conjugated polymer sandwiched between a pair ofelectrodes, one of which is transparent and the other of which is of amaterial suitable for injecting holes or electrons into the polymerlayer. The polymer material can be fabricated using a PVD process, orsimply by a spin coating technique using a solution of a solubleconjugated polymer. Ink-jet printing may also be used. Organicelectroluminescent materials exhibit diode-like I-V properties, so thatthey are capable of providing both a display function and a switchingfunction, and can therefore be used in passive type displays.Alternatively, these materials may be used for active matrix displaydevices, with each pixel comprising a display element and a switchingdevice for controlling the current through the display element.

Display devices of this type have current-addressed display elements, sothat a conventional, analogue drive scheme involves supplying acontrollable current to the display element. It is known to provide acurrent source transistor as part of the pixel configuration, with thegate voltage supplied to the current source transistor determining thecurrent through the display element. A storage capacitor holds the gatevoltage after the addressing phase.

FIG. 1 shows a known pixel circuit for an active matrix addressedelectroluminescent display device. The display device comprises a panelhaving a row and column matrix array of regularly-spaced pixels, denotedby the blocks 1 and comprising electroluminescent display elements 2together with associated switching means, located at the intersectionsbetween crossing sets of row (selection) and column (data) addressconductors 4 and 6. Only a few pixels are shown in the Figure forsimplicity. In practice there may be several hundred or more rows andcolumns of pixels. The pixels 1 are addressed via the sets of row andcolumn address conductors by a peripheral drive circuit comprising arow, scanning, driver circuit 8 and a column, data, driver circuit 9connected to the ends of the respective sets of conductors.

The electroluminescent display element 2 comprises an organic lightemitting diode, represented here as a diode element (LED) and comprisinga pair of electrodes between which one or more active layers of organicelectroluminescent material is sandwiched. The display elements of thearray are carried together with the associated active matrix circuitryon one side of an insulating support. Either the cathodes or the anodesof the display elements are formed of transparent conductive material.For downward emitting arrangements, the support is of transparentmaterial such as glass and the electrodes of the display elements 2closest to the substrate may consist of a transparent conductivematerial such as ITO so that light generated by the electroluminescentlayer is transmitted through these electrodes and the support so as tobe visible to a viewer at the other side of the support. Typically, thethickness of the organic electroluminescent material layer is between100 nm and 200 nm. Typical examples of suitable organicelectroluminescent materials which can be used for the elements 2 areknown and described in EP-A-0 717446. Conjugated polymer materials asdescribed in WO96/36959 can also be used.

For upward emitting arrangements, the substrate can be opaque orcoloured.

FIG. 2 shows in simplified schematic form a known pixel and drivecircuitry arrangement for providing voltage-addressed operation. Eachpixel 1 comprises the EL display element 2 and associated drivercircuitry. The driver circuitry has an address transistor 16 which isturned on by a row address pulse on the row conductor 4. When theaddress transistor 16 is turned on, a voltage on the column conductor 6can pass to the remainder of the pixel. In particular, the addresstransistor 16 supplies the column conductor voltage to a current source20, which comprises a drive transistor 22 and a storage capacitor 24.The column voltage is provided to the gate of the drive transistor 22,and the gate is held at this voltage by the storage capacitor 24 evenafter the row address pulse has ended. The drive transistor 22 draws acurrent from the power supply line 26.

The drive transistor 22 in this circuit is implemented as a PMOS TFT, sothat the storage capacitor 24 holds the gate-source voltage fixed. Thisresults in a fixed source-drain current through the transistor, whichtherefore provides the desired current source operation of the pixel.

The above basic pixel circuit is a voltage-addressed pixel, and thereare also current-addressed pixels which sample a drive current. However,all pixel configurations require current to be supplied to each pixel.

In a conventional pixel configuration, the power supply line 26 is a rowconductor, and is typically long and narrow. The displays are typicallybackward-emitting, through the substrate carrying the active matrixcircuitry. This is the preferred arrangement because the desired cathodematerial of the EL display element is opaque, so that the emission isfrom the anode side of the EL diode. This desired cathode material is ametal of low work function, such as calcium or barium, for the abilityto inject electrons with low applied voltages. These low work functionmetals are also reactive, so that it is not desirable to place themagainst the active matrix circuitry, as the required patterning is thendifficult.

Metal row conductors are formed, and for backward emitting displays theyneed to occupy the space between display areas, as they are opaque. Forexample, in a 12.5 cm (diameter) display, which is suitable for portableproducts, the row conductor may be approximately 11 cm long and 20 cmwide. For a typical metal sheet resistance of 0.2 Ωsquare, this gives aline resistance for a metal row conductor of 1.1 kΩ. A bright pixel maydraw around 8 μA, and the current drawn is distributed along the row. Arow of 1920 pixels (640×3 colours) can experience a voltage drop ofaround 9 volts. This can be reduced by a factor of 4 by drawing currentfrom both ends of the row, and further still by improvements inefficiency of the EL materials. Nevertheless significant voltage dropsare still present. This problem is worsened for larger displays, even ifthe total line resistance can be kept the same. This is because thereare more pixels per row, or alternatively larger pixels if theresolution is the same. The voltage variations along the power supplyline alter the gate-source voltage on the drive transistors, and therebyaffect the brightness of the display, in particular causing dimming inthe center of the display (assuming the rows are sourced from bothends). Furthermore, as the currents drawn by the pixels in the row areimage-dependent, it is difficult to correct the pixel drive levels bydata correction techniques.

ITO is used to form the common electrode in conventional displays, andhas a typical sheet resistance of 20/square. This increased sheetresistance makes ITO unsuitable for row conductors.

According to the invention, there is provided an active matrixelectroluminescent display device comprising an array of display pixels,each pixel comprising:

an electroluminescent display element; and

active matrix circuitry including a drive transistor for driving acurrent through the display element,

wherein the drive transistor and the display element are connected inseries between a power line for supplying or drawing a controllablecurrent to or from the display element and a common potential line, and

wherein the power line and the common potential line each comprise asheet electrode shared between all pixels of the array.

In this arrangement, sheet electrodes are used both for the currentsupply to the EL display element and the current sink. The power supplyline is thus a shared sheet electrode instead of the conventional rowelectrode. This reduces considerably the line resistance. For example,if the aspect ratio of the sheet electrode power line is 1:1 (square)the line resistance is simply equal to the resistance per square of thematerial, for example 0.2 Ω typically for metals. If the sheet isconnected around the entire periphery, the line resistance is lower.

The device comprises a substrate, and the active matrix circuitry mayoverlie the substrate with an electroluminescent layer overlying theactive matrix circuitry.

The display may be backward emitting through the substrate, so thatconventional materials may be used in the manufacture of the device. Thepower line may then comprise a substantially transparent electricallyconductive sheet (for example ITO) between the substrate and the activematrix circuitry. This provides a transparent layer, and the thicknesscan be selected to provide the desired resistance.

In this embodiment, an insulating layer is provided between thesubstantially transparent electrically conductive sheet and the activematrix circuitry, with contact portions provided through the insulatinglayer. This provides the contact from the power line (sheet) to eachpixel.

The display may instead be upward emitting away from the substrate, andmaterials have been developed which enable the EL display elementcathode to overlie the active matrix circuitry, with light output fromthe anode away from the substrate. In this case, the power linecomprises a metal sheet between the substrate and the active matrixcircuitry.

In this case, the common potential line needs to be transparent, andthus may comprise an ITO layer forming the anodes of the EL displayelements, and overlying the electroluminescent layer.

In another example, the display may instead be upward emitting away fromthe substrate, and with emission through the cathode. Cathode designshave been implemented which combine the required electrical propertieswith suitable optical transparency. In this case, the cathodes can formthe common potential line.

For example, the cathode may comprise a substantially opticallytransparent conducting layer of a first thickness, and a second layer ofa second, smaller thickness, and which comprises a low work functionmetal.

In these examples, one of the common potential line and the power supplyline needs to be transparent (as they are on opposite sides of the ELmaterial layer), and this will normally result in the use of ITO. Thishas a high resistance and it would be preferable to provide metallicelectrodes for both current carrying lines.

Therefore, in another example, a second metal layer is provided betweenthe substrate and the active matrix layer, isolated from the first metallayer, and wherein the second metal layer is connected to the commonpotential line. In this way, the common potential line and the powersupply line are connected to metal sheet layers.

The common potential line may comprise a substantially transparentelectrically conductive layer (e.g. ITO) forming the anodes or cathodesof the EL display elements (in the latter case, a low work functionmetal is combined with the ITO layer), and overlying theelectroluminescent layer, and the second metal layer contacts the commonpotential line with contact portions extending through the active matrixcircuitry. These contact portions also extend through openings in themetal sheet.

The invention will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 shows a known EL display device;

FIG. 2 is a simplified schematic diagram of a known pixel circuit forcurrent-addressing the EL display pixel;

FIG. 3 shows a simplified schematic diagram of a pixel circuit of theinvention;

FIG. 4 shows a first example of display device according to theinvention;

FIG. 5 shows a second example of display device according to theinvention;

FIG. 6 shows a third example of display device according to theinvention;

FIG. 7 shows a fourth example of display device according to theinvention; and

FIG. 8 shows a fifth example of display device according to theinvention.

It should be noted that these figures are diagrammatic and not drawn toscale. Relative dimensions and proportions of parts of these figureshave been shown exaggerated or reduced in size, for the sake of clarityand convenience in the drawings.

The invention provides a display in which the current is supplied anddrained using substantially continuous sheets of conductive material. Asshown schematically in FIG. 3, the power supply line 26 is a sheetelectrode, and it is shared between all pixels of the display. The term“line” will continue to be used, although of course the conductor is arectangular sheet. This power supply line is typically provided with afixed voltage, for example 8 Volts.

The invention can be applied to any pixel design, not only the basicvoltage-addressed pixel configuration of FIG. 3. Indeed, any pixel whichdraws current from a power line can be modified to take advantage of theinvention. There are many other types of pixel configuration, forexample including additional circuit elements for aging compensation orfor threshold voltage compensation, and there are also numerouscurrent-addressed pixel designs. These all require current to beprovided to the display elements of the pixels and can thereby benefitfrom the invention. These pixels may use NMOS, PMOS or CMOStechnologies.

FIG. 4 shows a first example of display device according to theinvention.

The device comprises a substrate 30, active matrix circuitry 32 and anelectroluminescent (EL) layer 34 over the active matrix circuitry 32.The substrate is typically glass. The active matrix circuitry is shownschematically as it may take various forms, as discussed above. Thedisplay is backward emitting through the substrate as shown by arrows36, and the common potential line (the earth connection in FIG. 3) isprovided by a metal cathode layer 38 overlying the EL layer 34. Theanodes of the individual pixels are defined by ITO pixel electrodes 40which in practice form part of the active matrix circuitry 32.

The power supply line 26 is implemented as an ITO sheet 42 beneath theactive matrix circuitry 32, in particular between the substrate 30 andthe active matrix circuitry 32. This provides a transparent layer, andthe thickness can be selected to provide the desired resistance.Furthermore, additional metal conductor portions may be provided outsidethe pixel areas to reduce the resistance further.

An insulating layer 44 is provided between the ITO sheet 42 and theactive matrix circuitry, with contact portions 46 provided through theinsulating layer 44. This provides the contact from the ITO power lineto each pixel. The insulating layer can be any suitable transparentmaterial, but is preferably silicon oxide or silicon nitride. Thecontact portions 46 are typically metallic contacts.

The display may instead be upward emitting away from the substrate, andmaterials have been developed which enable the EL display elementcathode to overlie the active matrix circuitry despite the low workfunction metal within the cathode layer. In this case, the light outputfrom the anode is away from the substrate. Upward emitting arrangementscan be made with higher aperture and also do not suffer transmissionloss through the supporting substrate. FIG. 5 shows an implementation ofthe invention for an upwardly emitting display. The same referencenumerals are used as in FIG. 4 for the same components.

In this case, the common potential line 38 is a transparent ITO layerand forms the anodes of the EL display elements. In this case, the pixelelectrodes 40 can be metallic. The power supply line is in this caseimplemented as a metallic sheet conductor 42, again with contactportions 46 through an insulating layer 44 making contact with theindividual pixel circuits of the active matrix circuitry 32.

In these two examples, one of the common potential line 38 and the powersupply line 42 needs to be transparent, as they are on opposite sides ofthe EL material layer 34. The use of ITO results still in a highresistance, even if the resistance is reduced by providing a metallicoverlay outside the pixel areas. The sheet conductors, both the powersupply line and the common potential line, have to carry the currentsfrom all pixels, rather than from only a row of pixels. The benefit ofthe reduction in resistance can be considered to be W/S where W is therow line width and S is the row spacing. In other words, for each row,the row conductor width is effectively increased from the previous rowwidth to the full spacing between adjacent rows. This ratio maytypically be around 0.2. Therefore, a benefit is only obtained if thematerials needed for the shared conductor have resistance no worse than5 times the metal resistance previously used for the metal rowconductors.

In practice the use of ITO limits the benefit of the examples of FIGS. 4and 5.

FIG. 6 shows a modification to the example of FIG. 5, and the samereference numerals are again used for the same components. A secondmetal layer 50 is provided between the substrate 30 and the activematrix layer 32, isolated from the first metal layer 42 (which is thepower supply line). The second metal layer 50 is connected to the ITOcommon potential line 38 overlying the EL material layer 34. In thisway, the ITO common potential line is connected at each pixel location(or at least at regular intervals) to a metal sheet layer. The powersupply line is also formed by a metal conductor 42 as in the example ofFIG. 5.

The second metal layer 50 is provided over the substrate 30 and a firstinsulator layer 52 overlies the second metal layer 50. The metal sheet42 of the power supply line 26 overlies the first insulator layer 52. Asecond insulating layer 44 (corresponding to the insulating layer 44 inFIG. 5) is provided between the metal sheet 42 and the active matrixcircuitry 32, with the contact portions 46 through the second insulatinglayer 44.

The connections between the second metal layer 50 and the ITO commonpotential line 38 comprise contacts 60 extending through openings 62 inthe metal sheet 42 and through the active matrix circuitry 32. The twometal sheets 42,50 should be as thick as possible and of as lowresistivity metal as possible. The dielectric spacers (the insulatinglayers) are high integrity material. The contacts 60 may be metal onlywith a patterned contact pad beneath or above the ITO layer 38, as it iseasier to pattern these contacts from metal rather than from ITO.

In the examples above, the light emitting display elements have opaquecathodes, and the light emission is from the anode side of the displayelement.

There are also pixel designs which enable transparent (or at leastsemi-transparent) cathodes to be formed. This then enables an upwardemitting structure to be formed while still avoiding patterning of thecathode. FIG. 7 shows one application of the invention to this type ofpixel design.

In FIG. 7, the same reference numerals are used as in FIG. 5 to denotethe same components. The anode overlies the active matrix substrate, andis again formed from an ITO pixel electrode 70. ITO is still used as theanode material even though there is no need for the anode to betransparent, because ITO has the required electronic properties (inparticular for the injection of holes) as well as deposition properties.A metal pixel electrode 72 lies beneath the ITO layer 70 and acts as areflector so that (substantially) all light emission is upwardly, awayfrom the substrate.

The cathode is formed from an ITO support 74 beneath which a thin layer76 of the desired cathode material is formed. Again, this cathodematerial includes a low work function metal such as calcium or barium.However, the thickness is reduced to a level where the cathode is atleast partially transparent, for example 50% transparent. Patterning ofthe cathode material is still avoided.

The example of FIG. 6, in which the resistance of the transparent topelectrode is reduced, can also be modified to use a transparent cathodevariation, as shown in FIG. 8.

In FIG. 8, the same reference numerals are used as in FIGS. 6 and 7 todenote the same components.

The anode again overlies the active matrix substrate, and again has anITO part 70 and a metal part 72. The cathode is again formed from an ITOsupport 74 and a thin layer 76 of the desired cathode material isformed.

There will be a number of possible materials for the conducting layerused to implement the invention. Where it is required to be transparent,ITO is an obvious candidate. Other transparent conductive layers havehowever been proposed. Where it can be opaque, the layer may be anysuitable metal layer, which may be formed as a foil or may be depositedby more conventional techniques.

The specific choice of materials to be used within the device structurehas not been discussed in detail in this application, as theimplementation of the invention will be routine for those skilled in theart. Indeed, the layers used conventionally in the manufacture of ELdisplay devices can be employed, and the invention essentially requiresrow conductors to be replaced by a continuous layer (possibly with vias)and with interconnections provided to individual pixels. The detailedprocessing steps will not therefore be discussed in detail.

Difficulties may arise in stepping the ITO layer 38 of FIG. 6 throughdeep contact vias to the underlying metal. A strap contact may be usedin this example, namely an additional metal layer above the ITO whichpasses through the via to the underlying metal electrode. The strapmetal may be in contact with the ITO or separated from it by aninsulator.

In the upward emitting examples of FIGS. 5 to 8, the substrate 30 may bea metal substrate which can then additionally function as one of thesheet metal contacts (for example avoiding the need for additionallayers to define the contacts 42 or 50.

Various other modifications will be apparent to those skilled in theart.

1. An active matrix electroluminescent display device comprising anarray of display pixels, each pixel comprising: an electroluminescentdisplay element (2); and active matrix circuitry including a drivetransistor (22) for driving a current through the display element (2),wherein the drive transistor (22) and the display element (2) areconnected in series between a power line (26) for supplying or drawing acontrollable current to or from the display element (2) and a commonpotential line, and wherein the power line (26) and the common potentialline each comprise a sheet electrode shared between all pixels of thearray.
 2. A device as claimed in claim 1, comprising a substrate (30),the active matrix circuitry overlying the substrate and anelectroluminescent layer (34) overlying the active matrix circuitry. 3.A device as claimed in claim 2, wherein the display is backward emittingthrough the substrate (30), and wherein the power line comprises asubstantially transparent electrically conductive sheet (42) between thesubstrate (30) and the active matrix circuitry.
 4. A device as claimedin claim 3, wherein an insulating layer (44) is provided between thesubstantially transparent electrically conductive sheet (42) and theactive matrix circuitry, contact portions (46) being provided throughthe insulating layer (44).
 5. A device as claimed in claim 2, whereinthe display is upward emitting away from the substrate.
 6. A device asclaimed in claim 5, wherein the power line comprises a metal sheet (42)between the substrate (30) and the active matrix circuitry.
 7. A deviceas claimed in claim 6, further comprising a second metal layer (50)between the substrate (30) and the active matrix layer, isolated fromthe first metal layer (42), and wherein the second metal layer isconnected to the common potential line (38).
 8. A device as claimed inclaim 7, wherein the common potential line comprises a substantiallytransparent electrically conductive layer (38) forming the anodes of theEL display elements, and overlying the electroluminescent layer (34),and wherein the second metal layer (50) contacts the common potentialline with contact portions (60) extending through the active matrixcircuitry.
 9. A device as claimed in claim 7, wherein the commonpotential line comprises a substantially transparent electricallyconductive layer (74) and a metal layer (74) forming the cathodes of theEL display elements, and overlying the electroluminescent layer (34),and wherein the second metal layer (50) contacts the common potentialline with contact portions (60) extending through the active matrixcircuitry.
 10. A device as claimed in claim 8 or 9, wherein the secondmetal layer (50) overlies the substrate (30), a first insulator layer(52) overlies the second metal layer, and the metal sheet (42) overliesthe first insulator layer (52).
 11. A device as claimed in claim 10,wherein a second insulating layer (44) is provided between the metalsheet (42) and the active matrix circuitry, contact portions beingprovided through the second insulating layer (44).
 12. A device asclaimed in claim 10 or 11, wherein contact portions which connect thesecond metal layer (50) to the common potential line (38; 74,76) extendthrough openings in the metal sheet (42).
 13. A device as claimed inclaim 6, wherein the common potential line comprises an ITO layer (38)forming the anodes of the EL display elements, and overlying theelectroluminescent layer (34).
 14. A device as claimed in claim 13,wherein an insulating layer (44) is provided between the metal sheet(42) and the active matrix circuitry, contact portions (46) beingprovided through the insulating layer.
 15. A device as claimed in claim5, wherein the substrate comprises a metal sheet which forms the powerline.
 16. A device as claimed in claim 5, 6, 7 or 9, wherein theelectroluminescent display element anodes are adjacent the substrate(30) and the light emission is through the cathodes.
 17. A device asclaimed in claim 16, wherein the cathodes form the common potential line(74,76).
 18. A device as claimed in claim 16 or 17, wherein the cathodecomprises a substantially optically transparent conducting layer (74) ofa first thickness, and a second layer (76) of a second, smallerthickness, and which comprises a low work function metal.
 19. A deviceas claimed in any preceding claim, wherein the active matrix circuitryfurther comprises, for each pixel, an address transistor (16) connectedbetween a data signal line (6) and an input to the pixel.
 20. A deviceas claimed in claim 19, wherein the active matrix circuitry furthercomprises, for each pixel, a storage capacitor (24) connected betweenthe power line (26) and the gate of the drive transistor (22).